I think I've got the back angle under control now. The session before this I still had a bit of rounding on some reps, but this session, I think I nailed it. My lower back was lit up like a christmas tree immediately after this set. It felt great.
What helped was fully extending my legs after each rep and bringing my knees to the bar from the same consistent position. I was assuming I didn't need to do that before, but it made the back positioning look very consistent and acceptable to me.
I'm trying to make sure taking my breath is the final step before I start the pull. My body keeps wanting to take a breath a second before I've finished setting my back.
https://www.youtube.com/watch?v=C_B0RQPi8iU